Polyester polymers and especially polyethylene terephthalate polymer are widely used for various applications, such as sheets, boards, extrusion blow molded bottles, extruded laminates, containers, and beverage bottles. The physical characteristics that make polyester polymers and polyester polymer particles such as polyethylene terephthalate (PET) desirable for packaging applications include impact strength, moldability, clarity, transparency, and low color. However, depending upon the specific application, there are other characteristics and properties that are desirable especially for stretch blow molded articles such as CSD and water bottles.
For example, one normally desirable feature of polyester polymer melts and solid particles (e.g. pellets) derived thereof is relatively high molecular weight, generally expressed as inherent viscosity (“IhV) or intrinsic viscosity (“It.V.”). To achieve high values of It.V., one known technique is to employ solids polymerization (i.e. “solid stating”). In general, solid stating is a process by which the average molecular weight of polyester polymer solids is increased. A certain minimum level of crystallization is a prerequisite for solid-stating because otherwise the solid particles would stick to each other at solid-stating temperatures. During solid-stating, crystallization continues both in terms of increasing percentage/crystallinity and increasing the perfection of the crystals, which manifests itself as increasing melting point. For example, partially crystallized PET pellets may be subjected to temperatures near but below the crystalline melt temperature for up to 12 hours in a fluidized bed allowing the PET pellets to increase their It.V. while the PET crystallinity increases as well. An inert gas flow or vacuum may be used to remove compounds that are volatile at solid-stating temperatures including acetaldehyde present in the solid polyester particles. Although desirable to eliminate solid stating, the absence of solid stating makes removal of acetaldehyde problematic. The situation is further complicated by the presence of acetaldehyde precursors which may later generate acetaldehyde when the polyester particles are melted (e.g. during injection molding of PET bottle performs). During solid-stating, there is some reaction of AA precursors, such as VEG with hydroxyethyl end groups (HEG) or water, to liberate AA, which may be partially swept away by the inert gas or vacuum. Without solid stating, acetaldehyde precursors may remain at the concentration present after melt-phase polycondensation. In addition, when solid-stating is planned, AA precursors are often present in lower amounts due to the shorter residence time in the melt phase.
Another normally desirable feature is a low concentration of acetaldehyde (“M”). Acetaldehyde has a noticeable taste and can be highly undesirable in beverage container applications. Two categories of AA are of known concern. The first is residual or free AA contained in polyester pellets or polyester particles used as raw material in injection molding or extrusion blow molding. A second type of AA is preform AA or the AA generated when PET pellets are melt processed to make bottle preforms. AA precursors in the solid polyester particles, chemical compounds or chemical functional groups which may react upon melting of the polyester, can produce unacceptable levels of AA in the preforms. In addition, new AA precursors are formed when the polyester polymer is held in the molten state, as in the case of an injection molding process to make bottle preforms. When performs are blown into bottles, unacceptably high AA levels are those that adversely impact the taste of the beverage contained in these bottles. Relatively tasteless beverages such as water are particularly negatively impacted by the taste of AA. Many water bottle applications require lower levels of preform AA than carbonated soft drink (“CSD”) bottle applications. Converters who take polyester particles and make bottle preforms would like to have one resin that could be used to make preforms for both water and CSD applications. This would simplify the materials handling process at the converter by allowing for one feed silo or one type of feed silo, one product storage area or one type of product storage area etc. . . . Most resins sold into water bottle markets have a lower It.V. than those resins sold into CSD markets. A dual use resin would have to a high enough It.V. for CSD applications and a low enough AA generation rate upon melting for water bottle applications.
There are a number of methods by which to address the problem of high residual AA levels in the solid polyester particles and/or high AA generation rates upon melting. For example, co-pending application Ser. No. 11/229,367, filed Sep. 16, 2005, discloses a process for producing polyester polymer, more specifically a process for producing PET polymers, wherein addition of various types of amine salts of phosphorus-containing acids to molten titanium-catalyzed polyester with a relatively high It.V can produce polyester polymers with low residual AA and low AA generation rates. Alternatively or in addition to other methods, converters may add AA scavengers to CSD resins to get acceptable perform AA for the water market. AA scavengers add significant cost to the container and often negatively impact the color of the container by making it either more yellow or darker as compared to an analogous container without AA scavenger added. When certain AA scavengers are used, the level of black specks present in the solid polyester particles and/or in the molded part can also increase, which results in an undesirable increase in the number of black specks in subsequent molded products.
Another example of a normally desirable characteristic of the polyester polymer melts and any subsequent polyester particles produced by solidification of the melt is that of low vinyl ends concentration. Vinyl ends as represented by the formula: —CO2—CH═CH2 are known AA precursors. One commonly accepted mechanism by which AA is generated in molten polyester is the internal chain scission of a polyester polymer chain to form a vinyl end group and a carboxylic acid end group. The VEG can react with a HEG to form residual or free AA and a new internal ester linkage. There is a common perception that a high concentration of vinyl ends is thus undesirable due to the ability of the vinyl end to react to form AA during subsequent melt processing of the polyester polymer.
Further, U.S. Pat. No. 5,852,164 indicates that the concentration of olefin terminals or end groups, which is the sum of the vinyl ends, the vinylidene ends, and the methyl cyclohexene ends, is preferred to be less than 25 eq/ton in order to improve the melt heat stability of highly modified polyester polymers, which contain in almost all of the examples about 33 mole percent of 1,4-cyclohexanedimethanol, based on a total diol content of 100 mole percent. In general, it is undesirable, especially in molding processes, for the intrinsic viscosity of the polymer to decrease significantly upon melting as the properties of the resulting article or part will be negatively impacted. Additionally, it is known that vinyl ends may also polymerize under extreme conditions to polyvinyl esters which may eliminate to form poly(enes) that may be responsible for yellow coloration of PET.
Because vinyl ends are known AA precursors, there is a general tendency to operate melt phase polyester polymerization processes at temperatures and production rates to inhibit subsequent generation of AA in downstream melt processing applications. This is especially true when a precursor It.V. is made in the melt phase, followed by solid-stating to obtain a product It.V., which is acceptable for a given application.
What is not generally appreciated is that what is important for AA generation upon melting is not the fact that the VEG concentration is relatively high but why or how the VEG concentration was increased. If the VEG level is relatively high due to an increased finisher temperature with all other things being equal, then the level of AA generated upon melting a polyester will increase. If the VEG level is relatively high due to a decreased reaction rate for the conversion of VEG to AA with all other things being equal, then fewer VEG will be converted to AA with the result that the level of VEG will increase and the level of AA generated upon melting the polyester will decrease.
It is easier to influence the VEG to AA reaction rate, which occurs during melt processing, when the polyester is manufactured exclusively in the melt phase. This is because efforts to slow down the VEG to AA reaction rate after the melt phase manufacturing of polyester precursor usually also have a negative impact on the polycondensation rate during solid stating. On the other hand, it is possible to use a conventional process, including solid stating, and impact the VEG to AA reaction rate at the start of the injection molding process or in a prior extrusion step, such as compounding. This approach would usually be more costly and/or problematic than action taken at the end of a melt-phase line used to make a product or final It.V.
A polyester polymer with the properties of a relatively high vinyl ends concentration from higher temperatures and low AA generation rates is attractive from an economic perspective. For example, it would desirable to operate a PET production process at higher temperatures and higher throughput rates thereby allowing high vinyl ends concentrations to rise to higher concentrations than other known PET polymers while maintaining comparable AA generation rates in subsequent processing applications, such as blow molding of bottles. Efforts to slow down the VEG to AA reaction rate, which increases the level of VEG, will allow higher temperatures to be used in the melt phase manufacturing, which also increases the VEG level, and still obtain lower levels of AA generated during molding as compared to the analogous case at higher temperature but without the VEG to AA reaction rate slowing efforts. Slowing down the VEG to AA reaction rate where the manufacturing temperature is hotter may result in more AA generated or a higher preform AA than in an analogous case with a cooler finisher temperature. It should be noted that to obtain very low preform AA values, it may be necessary to use low to moderate manufacturing temperatures and low to moderate catalyst concentrations in conjunction with slowing down the VEG to AA reaction rate.
Hence, there is a need for a polyester polymer with a high It.V. produced entirely in the melt phase that avoids the costly step of solid stating. Further, the polyester polymer could be treated near the end of the entirely melt-phase manufacturing process so that the VEG to AA reaction rate slows down; therefore, the level of VEG in the solid polyester polymer particles increases while the level of AA generated during melt processing or the preform AA decreases, relative to the case with no treatment. In one embodiment, the polyester polymer could be produced at higher temperatures and higher throughputs thereby resulting in relatively high vinyl ends concentrations and then be treated near the end of the entirely melt-phase manufacturing process so that the VEG to AA reaction rate slows down thereby resulting in relatively high vinyl ends concentrations, yet still generate low amounts of AA upon remelting in the absence of AA scavengers, relative to the case with no treatment. In another embodiment, the need is especially great in resins for water bottle applications which normally demand low levels of M in performs and bottles, and in these cases, the manufacturing temperatures would be low to moderate, in conjunction with the treatment to lower the VEG to AA rate. It would be even more desirable if the same polyester can be utilized as raw material for both carbonated soft drink and water bottle applications.